Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 301835
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
235 PM EDT Sun Apr 30 2017

.SYNOPSIS...
A cold front will approach the Western Carolinas later tonight and
move across the forecast area on Monday with strong to severe
thunderstorms possible as it does. Dry, weak high pressure builds
across the southeast Tuesday through Wednesday. On Thursday and
Friday, a moist area of low pressure will lift across the region
from the southwest.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 2:30pm EDT Sunday: A few elevated showers have popped-up in
the last hours.  These showers are moving almost due north and have
had no lightning so far.  With CAPE analyzed at 500 to 1000 j/kg, it
is possible one or two of these showers will intensify into a
thunderstorm this afternoon before this diurnal activity fades at
sunset.

Due to more extensive cloud cover, highs are running around 5
degrees below yesterday, though did increase temps a degree or so
with this update as clouds were opening some.  Reduced temperatures
will limit CAPE and the chances for any significant precip
today, but precip. becomes likely as major front approaches late
tonight and tomorrow from the west.

Area is currently in a well-defined and large warm sector ahead of
major frontal system currently stacked over the south-central
CONUS.  Cold front with line of convection is currently moving
through LA into MS.  Latest timing has front into the GSP CWA early
Monday morning, passing through by late Monday afternoon.
Significant precipitation/thunder is expected near and along the
front.  Passage of cold front will cool temps and scour moisture,
ending immediate chances for precip. Main question with this event
will be the chances for severe weather.  Instability and wind shear
are both in the moderate range with 30 to 40 kts of bulk shear and
LI to -5 Monday afternoon.  Shear is much better further north, but
combination of ingredients should be enough for at least some severe
weather on Monday, with wind gusts in bowing segments of expected
QLCS-type system.  An isolated, brief tornado is not out of the
question, but alignment of shear direction with front orientation
favors QLCS and non-super-cellular storms.

850mb flow will also be quite strong with passage of the front, with
southerly to southwesterly winds of 40 to 55 kts from tonight
through Monday.  Surface winds are not expected to reach advisory
levels, except possibly at the highest elevations, and have declined
to issue a product for wind at this time.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
As of 120 PM EDT Sunday: a considerably more tranquil weather
scenario is expected for the middle of the week. The back edge
of the precip associated with the cold front might still be over
the eastern-most zones at sunset on Monday, but that should move
away to the east in short order. The upper low lifting NE of the
Gt Lakes on Monday night should allow for the upper trof to the
west to deamplify through Tuesday, resulting in a nearly zonal
upper pattern thru Tuesday night. With that, we welcome weak high
pressure that should bring sunny sky, normal temperatures, and a
dry air mass that will result in a nearly perfect spring day on
Tuesday. The weak high will move to our east on Wednesday as the
upper pattern buckles with the next upper trof digging over the Srn
Plains. That should start a return flow of moisture from the Gulf
ahead of the system late in the day. However, present indication
in the model data is that moisture/precip will not arrive before
the end of the daytime period on Wednesday. That should give us
another mostly dry day with some increasing clouds in the afternoon,
and temps creeping up a bit above normal.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 235 PM EDT Sunday: still looking at the development of
another weather system for the late part of the week, but right
now that system remains a moving target. The main issue appears to
be when/where the upper trof cuts off a low.  Will it be Thursday
morning over the Ozark Plateau per the GFS, or the lower OH Valley
Thursday night as in the new ECMWF? Either way, we are looking at
significant moisture transport from the Gulf of Mexico ahead of the
system and lifting northeast across the western Carolinas beginning
Wednesday night. See no problem as the models agree in principle
of providing a wet period centered mainly on Thursday/Thursday
night, so precip probability is ramped up to the likely range
across the whole fcst area. Have resisted the temptation to go
categorical for now, given the run-to-run consistency issues with
the details of this system. The main issue with the guidance creeps
in on Friday and that deals with how quickly the system moves
past. The op GFS remains the slower solution with the center of
the low moving slowly from the Cumberland Plateau to the Central
Appalachians Friday through Saturday morning. Even if the slower
solution comes to pass, most of the area east of the mtns would
develop a downslope flow Friday night that would limit additional
precip, while a more NW flow would enhance or maintain precip
chances near the TN border. The precip probs will be moved in that
direction. Temps should stay below normal Thursday through Saturday
with the extensive cloud cover and threat of precip. Expect the
system to lift out in time Sunday to salvage half the weekend,
with some clearing and temperatures returning to normal.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
At KCLT and elsewhere: A few widely scattered elevated convective
showers are expected this afternoon, with any activity declining
after sunset.  CIGs are expected to decline tonight and tomorrow to
MVFR or lower as very moist boundary layer cools. Main impacting weather
will be frontal system/squall line expected to pass through the area
Monday morning.  TAF timing for PROB30 -TSRA reflects latest CAM
timing of front location and line of precip.  With line entering
western aerodromes after 11Z and exiting eastern aerodromes by 18Z.
Winds will be persistently SSE to SSW with winds veering after FROPA
later on Monday.  Monday is expected to see some higher gusts as the
front passes by, reflecting 850mb winds of 40kts or more and
convective activity.

Outlook: Behind the cold front exiting by Monday afternoon, drier
weather is expected Tuesday. A return of afternoon/evening
thunderstorms is possible Wed, while another cold front may bring
another round of widespread showers and storms by Thursday into
Friday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  88%     Med   77%     High  92%     High  85%
KGSP       Med   68%     Med   75%     Med   63%     High  80%
KAVL       High  93%     Med   62%     Med   69%     High  83%
KHKY       Med   75%     Med   75%     Med   62%     Med   72%
KGMU       High  85%     Med   77%     Med   65%     High  80%
KAND       High  94%     High  88%     Med   60%     Med   69%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JPT
NEAR TERM...WJM
SHORT TERM...PM
LONG TERM...PM
AVIATION...WJM


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